1. Field of the Invention
The present invention relates to an image forming apparatus and an image forming method that are capable of accurately detecting the conveyance speed of an intermediate transfer belt.
2. Description of the Related Art
Generally, in an image forming apparatus, it is desirable to form an image at a desired position on a sheet. In the case of a color image forming apparatus that can form images of a plurality of colors, images of a plurality of colors are superposed on one another to form a color image. Accordingly, in order to reduce color misregistration, it is desirable to match the image formation positions of images of a plurality of colors. In an intermediate transfer type color image forming apparatus, toner images of a plurality of colors are formed on respective photosensitive drums. The toner images are sequentially transferred onto an intermediate transfer belt, and the multicolor images on the intermediate transfer belt are collectively transferred and fixed on a sheet, so that a color image can be obtained.
In such an intermediate transfer type color image forming apparatus, it is necessary to accurately superpose toner images of respective colors formed on the photosensitive drums on the intermediate transfer belt. However, if the speed of the intermediate transfer belt varies, misregistration of toner images of respective colors may occur. To solve the problem, techniques to detect the speed of an intermediate transfer belt and to correct the operating state of an apparatus have been proposed.
For example, Japanese Patent No. 3344614 discusses a technique to separately dispose two sensors with some distance in a conveyance direction of an intermediate transfer belt. The two sensors detect a mark, and, based on a time interval of the detection of the mark, the conveyance speed of the intermediate transfer belt is detected. Based on the detected conveyance speed, the driving speed of the intermediate transfer belt is controlled such that the conveyance speed becomes constant.
Further, in Japanese Patent Application Laid-Open No. 2005-156877, two sensors detect a mark at some time intervals. Based on the time intervals, the conveyance speed of an intermediate transfer belt is detected, and the conveyance speed obtained after the correction of color misregistration is stored. Then, a correction is performed to match the subsequent conveyance speed of the intermediate transfer belt with the stored conveyance speed.
However, the above-described techniques do not mention a specific value and reason about the interval of two sensors for detecting the conveyance speed of the intermediate transfer belt. In the case where the conveyance speed of an intermediate transfer belt is detected using one mark provided on the intermediate transfer belt and two sensors separately disposed with a distance in a conveyance direction, if the distance between the two sensors is not appropriately managed, a large speed detection error may occur.